Process for stabilizing dicalcium phosphate dihydrate



United States Patent 3,464,786 PROCESS FOR STABILIZING DICALCIUM PHOSPHATE DIHYDRATE Heinz Harnisch, Lovenich, near Cologne, Joseph Cremer,

Hermulheim, near Cologne, and Friedrich Schulte, Hurth, near Cologne, Germany, assignors to Knapsack Aktiengesellschaft, Knapsack, near Cologne, Germany, a corporation of Germany No Drawing. Filed May 18, 1966, Ser. No. 551,058 Claims priority, application Germany, June 16, 1965,

Int. Cl. C01b 25/32 U.S. Cl. 23-108 8 Claims ABSTRACT OF THE DISCLOSURE A process for stabilizing dicalcium phosphate dihydrate wherein less than a stabilizing amount of a stabilizing agent consisting of alkali metal phosphate or alkaline earth metal phosphate is added to the dihydrate in situ during preparation thereof from calcium salts and phosphoric acid; the remainder of said stabilizing agent being added after recovery of the dicalcium phosphate dihydrate and the stabilizing agent and dihydrate jointly ground; and preparations utilizing the specially treated dihydrate.

TABLE I CZHPOM addition, percent Material by Weight; Glycerol test Unstabilized DCP Insufficient. Stabilized DC 0 Very good.

Do 1 Do. Do Very good, good. Do. 10 Good. Do Satisfactory. Do 30 Insufficient.

The degree of stabilization is determined by means of the so-called glycerol test, wherein the DCP is pasted up with glycerol and water to give a just mobile paste, which is heated in the water bath for 30 minutes to a temperature of 100 C. The glycerol test is stated to be very good when, after that time, the paste is found to have an unchanged consistency. The result obtained must be considered as being insufficient when the paste is found to have hardened. The adjectives good, satisfactory, and sufficient are reserved to the characterization of transition grades lying between these two extremes.

The stabilization of DCP is indispensable when it is desired substantially to reduce dehydration. The following compounds have inter alia been proposed for use as stabilizing agents.

(1) Alkali metal pyrophosphates, e.g. tetrasodium pyrophosphate. They are used in a proportion of 0.5 to 1.25% by weight, referred to DCP, and they are prepared either separately in an aqueous solution to be added then to DCP, or are dissolved in the phosphoric acid necessary for preparing the DCP.

(2) Alkaline earth metal pyrophosphates, e.g. calcium "ice pyrophosphate and magnesium pyrophosphate. The proportion necessary to achieve stabilization is 5% by weight for calcium pyrophosphate, and is 3% by weight for magnesium pyrophosphate. The calcium pyrophosphate is added to the phosphoric acid necessary for{ making the DCP, whereas the magnesium pyrophosphate is prepared from MgO and pyrophosphate ions in the phosphoric acid.

(3) Alkali metal hexametaphosphates. l-qThese compounds are added in aqueous solution in a proportion of 0.3 to 3% by weight to the DCP in a separate operational step or during another operation, e,g, filtration.

(4) Alkali metal tripolyphosphates. These stabilizers are used in a manner analogous to the alkali metal hexametaphosphates.

(5) Mixtures of alkali metal and alkaline earth metal pyrophosphates, e.g. Ca P O +CaNa P O =4H Q Phosphoric acid placed into a reaction vessel is neutralized by means of calcium hydroxide at a pH-value of 5 to 7 and, during the neutralization, the calcium pyrophosphate is formed by adding 0.4 to 0.8% by. weight polyphosphoric acid containing about 40% H P O In addition thereto, the dried DCP is admixed then with 0.6 to 1.2% by weight CaNa P O -4H O. However, the results so obtained are unsatisfactory in view of the fact that homogeneous mixing of the small stabilizer proportions with DCP is impossible or possible only after extremely prolonged mixing periods, especially in the event of relatively large batches.

(6) Magnesium phosphates, e.g. trimagnesium phosphate and dimagnesium phosphate. Dirnagnesium phosphate is used in a proportion of 2 to 4% by weight, the trimagnesium phosphate is used in a proportion of 0.5 to 2% by weight. In either case, the stabilization is achieved by admixing the stabilizer with dry DCP.

It has now unexpectedly been found that the customary stabilization of the above dicalcium phosphate dihydrate by means of an alkali metal and/or alkaline earth metal phosphate can be substantially improved by initially adding a portion of the stabilizer to the dicalcium phosphate dihydrate in situ before recovery and thereafter adding the remainder to the recovered dihydrate crystals and grinding the dihydrate and stabilizer together.

Preferably 0.4 to 0.9% by weight, more preferably 0.8% by weight of the stabilizer, referred to the dicalcium phosphate dihydrate, is added to the dicalcium phosphate during preparation, and additionally 0.2 to 0.6% by weight, advantageously 0.4 by weight, of the stabilizer is added to isolated, solid dicalcium phosphate dihydrate, and ultimately ground jointly with the solid dicalcium phosphate.

Especially favorable results are obtained when the stabilizer portion to be added during the preparation of the dicalcium phosphate is added at a pH-value of 7.5 to 8.5, preferably at a pH-value of 8.0, and when the stabilizer portion to be added before the dicalcium phosphate is ground has a grain size approximately the same as that of the dicalcium phosphate.

The present invention has proved especially advantageous for making stabilized dicalcium phosphate dihydrate formed of grains with a size of less than 40% from coarsely crystalline dicalcium phosphate which consists to an extent of more than 30% by weight, e.g. of 70 to by weight, of grains having a size larger than 40,14. In this event, the dicalcium phosphate is most advantageously admixed, during its preparation, at a pH- value of 7.5 to 8.5, with 0.4 to 019% by weight of the stabilizer, referred to the dicalcium phosphate, and the isolated, solid dicalcium phosphate dihydrate formed to an extent of more than 30% by weight of grains having a size larger than 40 is admixed then with 0.2 to 0.6% by weight of the stabilizer, referred to the dicalcium phos- TABLE II S tahilizer N arlgOy Addition prior to grinding Stabilized in reactor at pH Glycerol test 1 1.2 Sufficient.

Satisfactory, sutlicient. Satisfactory. Satisfactory, suflicient. Satisfactory.

Geog, satisfactory.

0. Good. Very good.

Good

Addition in reactor Very good.

Do. Good. Insufficient. Satisfactory, insutiicicnt. Sufficient. Satisfactory. Very good.

l The data indicated in columns 1 an are percent by weight, referred to DC Example A stainless steel reactor having a capacity of 1 cubic meter and provided with a rapid propeller stirrer (1500 r.p.m.) was filled with 220 liters water. At the same time, 100 kg. CaCO were suspended in water in a stainless steel container; the CaCo -suspension had a density of about 1.3 to 1.5. In the first process step, the carbonate suspension and phosphoric acid having a strength of 70 to 85% by weight were pumped simultaneously into the reactor"; The supply of phosphoric acid was so controlled by means of a regulating valve that the pH-value varied between 2.3 and 2.6. As soon as all of the carbonate had undergone reaction, the pH-value was finally adjusted in a second process step while reacting the phosphoric acid in excess with calcium chloride (25-35% by weight strength) and with sodium hydroxide solution (20-50% by Wfiight strength) in stoichiometric quantitative proportions. The temperature in the reactor raised from initially 25-3Q C. (first step) to 40-45 C. The supply of calcium chloride and sodium hydroxide solution was arrested at a pH-value of 8.0. Thereafter 0.8% by "weight sodium pyrophosphate was added to achieve the stabilization. The finished DCP was filtered by means of a vacuum drum filter, and washed. The filter cake which contained about 40% by weight water of adhesion was dried in a twostage turbulent layer drier, admixed with 0.4% by weight sodium pyrophosphate, and ground in a sitter bowl mill to obtain grains of which 99.5% had a size of less than 40a. The sodium pyrophosphate added was formed of grains of which about to had a size larger than 40 i.e. a size approximately corresponding to that of the DCP before grinding.

We claim:

1. In a process for stabilizing dicalcium phosphate dihydrate with a stabilizer consisting of an alkali metal phosphate or alkaline earth metal phosphate, the improvement comprising:

(A) eflecting a preliminary treatment of the wet dicalcium phosphate dehydrate in situ during preparation and at a pH of about 7.5-8.5 with about .4% .9% by Weight of stabilizer;

(B) adding to the recovered dicalcium phosphate dihydrate additional stabilizer in an amount corresponding to about .2-.6% based on the amount of dicalcium phosphate dihydrate recovered, and

(C) grinding the recovered dicalcium phosphate dihydrate and stabilizer jointly.

2. The process of claim 1 wherein the (A) step is effected by adding about .8% by weight of stabilizer based on the amount of dicalcium phosphate dihydrate.

3. The process of claim 1 wherein the (B) step is effected with about .4% by weight of stabilizer based on the amount of dicalcium phosphate dihydrate.

4. The process of claim 1 wherein the (A) step is effected at a pH of about pH 8.

5. The process of claim 1 wherein the stabilizer added in step (B) has a grain size about the same as the dicalcium phosphate dihydrate.

6. The process of claim 1 wherein more than 30% of the dicalcium crystals in step (B) are larger than 40 and the (C) step is etfected to obtain combined grain size of less than 40g.

7. The process of claim 1 wherein the stabilizer is a member selected from the group consisting of tetrasodium pyrophosphate, calcium pyrophosphate, magnesium pyrophosphate, calcium-disodium pyrophosphate, alkali metal tripolyphosphates, alkali metal hexametaphosphates, dimagnesium phosphate, and trimagnesium phosphate.

8. A stabilized dicalcium phosphate dihydrate preparation containing dicalcium phosphate dihydrate and a stabilizing agent wherein said dicalcium phosphate diihydrate is initially treated in accordance with the process of claim 1.

References Cited UNITED STATES PATENTS 2,287,699 6/1942 Moss et a1. 23-l08 3,066,056 11/1962 Schlaeger et al 23-108 OSCAR R. VERTIZ, Primary Examiner H. J. MILLER, Assistant Examiner 

